It has long been suspected that there are underground tunnels and caverns on the moon, presumably the residua of lava tubes. Now a group of scientists led by Junichi Haruyama of the Japanese Aerospace Exploration Agency are reporting the discovery of a "skylight" leading into an underground cavern.

The hole measures 65 metres across, and based on images taken at a variety of sun angles, the hole is thought to extend down at least 80 metres. It sits in the middle of a rille, suggesting the hole leads into a lava tube as wide as 370 metres across... Since the tubes may be hundreds of metres wide, they could provide plenty of space for an underground lunar outpost. The tubes' ceilings could protect astronauts from space radiation, meteoroid impacts and wild temperature fluctuations...